It is known that in engines of the kind described with port injection controlled by measurement of the air flow, it is possible to obtain a relatively accurate control of the fuel-air ratio. This makes it possible to reduce the polluting components present in the exhaust gases under almost all conditions of operation.
However, it has now been found that at low temperatures and in the cold-running phase and as a consequence of the losses due to deposition of liquid fuel in the region around the injection nozzle with a cold inlet port, and also in the combustion chamber, it is necessary to enrich the mixture to balance out this effect. However, enrichment of the mixture makes the exhaust gas quality unsatisfactory under these operating conditions, at least until the operating temperature is reached and until therefore the fuel that is deposited in the inlet port is re-evaporated. However, it has also been found that even when the engine has reached its operating temperature slight deposits of liquid fuel can still take place in the inlet port and then when there are changes in the load which result in an increase in the depression in the induction system, these deposits evaporate suddenly and are picked up and lead to enrichment of the mixture and the undesired consequences already mentioned.